With the field of the packaging industry, where certain types of liquid contents, e.g. milk, are packed into consumer packages of the non-returnable type, packing machines are used, for example, which convert prefabricated packing container blanks into bottom-closed packing containers which subsequently are filled directly with the particular contents and closed. The packing container blanks are generally supplied to the machine in the form of flattened, tubular blanks which have to be raised before the conversion to individual packing containers, that is to say they must be converted to four-sided tubes of substantially square cross-sectional shape. This raising usually takes place in connection with the blank being transferred from a magazine at the inlet end of the packing machine to a first conversion station in the packing machine proper.